A devoted, successful husband — Sebastian Grigg, a partner in Goldman Sachs — five children, a job she loved, a beautiful house in London, wonderful family and friends, including David Cameron, Dominic West and Boris Johnson.

And yet behind the glittering façade, all was not well, although she didn’t quite know it herself until she descended, without warning, into a depressive episode so bad that she was bed-ridden, in terrible physical pain, for six months in 1997 after the birth of her second child.

The descent was vicious. One evening Rachel put her two young children to bed as usual, then spent the night wrestling with insomnia and anxiety over a million mundane things. Within days she was in the grip of constant dread — ‘we’re going to crash’ was her refrain, a feeling of hurtling towards disaster — and suffering pain so bad her family thought she was having a heart attack.

Rachel spent six months in bed, recovering slowly, with the help of antidepressants, being ministered to constantly by her mother and husband. She returned to work at the Times, where she was a feature writer and columnist, and another child. Six years later, after the birth of twins, she again dropped into depression that kept her bed-bound and despairing, for a whole year.

“It was as if I had been engaged in a struggle to try and cut off my own dark shadow. Now the shadow and I were reunited,” she writes in Black Rainbow, a moving account of her struggle to get better and stay well, that is also an anthology of carefully chosen poems that have been of help to Rachel in bleak times. There are passages from the Bible, prayers, poems by William Blake, George Herbert, Emily Dickinson and Yeats.

The book was launched by Boris Johnson — a childhood friend — who recited an unexpectedly good poem of his own. And Black Rainbow has been a success, climbing steadily through the bestseller charts in the UK, resting within the top ten for several weeks by the time I meet Rachel, on a sunny morning at the home of my friend, her cousin, Eliza Pakenham.

Rachel looks a teeny bit like a fair-haired Helena Bonham Carter, with a similar precise and delicate prettiness. She immediately leaps that slight, wary distance that can exist between interviewee and interviewer, determined to speak openly and truthfully. The book is warm-hearted and sincere in its attempts to offer hope and some kind of blueprint for wellness to sufferers from depression, but also an understanding of the disease to those who don’t.

“After the first breakdown, I was in denial,” she admits now. “There is still a taboo around depression. I went straight back to the office and tailored suits. Sebastian and I were both very busy and hard-working. The world was our oyster.

I didn’t really change my life at all, I didn’t have therapy. I didn’t want to know. I just thought ‘this was a very unpleasant episode, the less I talk about it, the better.’ So I shut it up altogether. Then, bang! I had the second episode. It was only then I was forced to change. The pattern with depression is, it’s like a watercolour. Each successive episode is deeper and darker.”

Brought up a Catholic, Rachel spent all her holidays as a child at her grandmother’s cottage in Wexford, and has “extraordinarily happy memories of those times. I feel very at home with the idea that Ireland, of all countries, celebrates poetry. I don’t want to claim to be more Irish than I am, but those childhood memories are very strong, and whenever I come back, I pick up those feelings. My faith as well, being brought up a Catholic, connects me.”

It was also in Ireland that she met Sebastian for the first time, at Tullynally Castle, the Westmeath home of her cousins, the Pakenhams. “We met when we were 17,” she recalls. “I had a very strong sense of him. I can remember exactly where he was standing when I first set eyes on him, and what he was wearing, white jeans and a white shirt.”

It was a meeting that made an instant impression; “I remember thinking, ‘ok, don’t blow it.’ I didn’t know the future, but I remember thinking, ‘don’t rush that one. Go carefully, that’s something rather special.’” Indeed it was special. The two didn’t start dating until the end of university – Oxford – “but we were really childhood sweethearts,” Rachel says.

The most frightening part of Rachel’s story is the way in which depression struck, without warning, despite all the good things. It wasn’t just the successful career, the privilege, the material wealth, it was the love, too. A loving birth family (her mother is one of the great heroines of the book), adored children, and the great gift of a loving husband; none of these “blessings” as she rightly calls them, was proof against the darkness.

For all that care and love did not prevent depression, they undoubtedly helped the return from it. The vigilance and devotion of Rachel’s mother, the solid determination of Sebastian, even in the face of their own dread. “The first time was especially terrifying, because they didn’t know that I would get better. In fact, Sebastian had planned a life in which I wouldn’t,” Rachel says now, “where he would be looking after the boys and I’d be upstairs like Mrs Rochester, screaming ‘I’m going to crash’.”

As she talks, hard memories hover at the edge of the conversation, still clear and frightening, but around them Rachel has put in so much vibrant and positive effort that they are held at bay, pushed to the outer reaches by her belief, now, in her ability to do the things that work for her. “After the second breakdown, I had this long period of being ill on and off. I knew then I had to find all these different strategies to get better.”

The strategies are many and various. “The thing about depression,” she explains, “is, there’s no magic bullet. I use lots of different approaches, and I get this great calm through words. The experts say your most successful strategy is a bit of everything. So a big motivation for writing the book was to share some of that. Poems are not for everybody, but if one person finds that holding on to a healing mantra helps, that’s good.”

She cannot, she says, “afford to break the rules. I have responsibilities to the children and myself.” Certainly, the saddest moments in the book are those in which Rachel, unable to leave her bed, lost in terror and pain, feels the guilt of her inability to be a mother. Does she still feel that? “The guilt comes and goes,” she admits. “I do on occasion feel very sad at the wasted years, especially when the children were little. But two things help; Sebastian’s reassurance that he and my family gave the children lots of cuddles and love when I couldn’t. And thinking that it is a blessing to understand more about mental health, and so I try and equip my children with good strategies to become emotionally resilient. I work with all of them, doing breathing, little mindfulness practises, a lot of poetry,” says Rachel. “The idea is to learn long before anybody is ill; don’t wait until you’re 30 and have a breakdown.”

Does she reflect on what it was in her character that led her to such a dark place? “I was an anxious person, a striving person” she says now, “but not a gloomy or unhappy person, and I wouldn’t describe myself as unhappy now. I feel very lucky, very blessed. But,” she laughs, “part of the therapy is I have had to learn to be more gentle and compassionate with myself. To look after myself, a bit like a nervous pet.

“We had a fabulously exciting life, lots of our friends were doing interesting things, I loved it all. But we did take choices as a couple that were demanding. Sebastian was working long hours at Goldman Sachs, and he did stand for parliament in 1997” – Sebastian, who appears in the famous Bullingdon Club photo at Oxford with David Cameron and Boris Johnson, stood as a Conservative MP – “I was in the newsroom, and we had two small children.

Something had to give. One definition of good mental health is integrity between the inner and outer life. From the outside, you can look like you’ve ticked all the boxes, and you have, but it may be at a huge cost to you in terms of how hard you’re paddling underneath to keep the show on the road. I was a cautionary tale,” she says. “A reminder that there is this thing called your health, and it might just jump up and say, ‘hang on a sec…’”

During the first depressive episode, Rachel latched onto a phrase from the Bible that her mother taught her: ‘My grace is sufficient for thee; my strength is made perfect in weakness,’ repeating it like a mantra. It was, in those early days, the only thing her mind could hold onto in the fight to stay out of the sickening darkness. It was, too, a moment of light and optimism – that even the worst was somehow within God’s plan.

That determination, to find hope has been a key part of Rachel’s recovery. “I wouldn’t wish it on anyone, it’s a hard way to learn, but if it hadn’t happened, I wouldn’t have been able to rethink my whole life. I was forced to slow down, to look at my own view of what it is to be successful.

I now work a lot in prisons and hospitals, and I think that’s just as valid as Sebastian’s career in finance. We are all united by our common humanity,” she insists. “Scratch the shiny surface and everybody is dealing and struggling with all sorts. Its actually very relaxing when you get to the point of realising that.”

Black Rainbow closes with Derek Mahon’s wonderful Everything Is Going To Be Alright: “The sun rises in spite of everything/ And the far cities are beautiful and bright… Everything is going to be alright.” That truth is something Rachel has learned to believe in passionately.

My Journey Through Depression, by Rachel Kelly is published by Yellow Kite, £16.99. All author proceeds go to mental health charities United Response and Sane.